How to manage the risk of heat stress

The human body is designed to best function within a specific range of internal temperature. This is referred to as our “core” temperature, and the range varies from 36°C to 38°C. The human body makes use of two cooling mechanisms to reduce and remove excess heat from the body and return our core temperature to below 38°C: 

• Heart Rate: When our bodies start to overheat, our heart rate will increase. The heart rate increases so that the blood and heat are moved from the heart, lungs, and other organs to the skin. 

• Sweating: Sweat is moisture exuded through the pores of the skin as a reaction to heat. This helps the blood and body to cool down. Evaporation of sweat is the most important way the body gets rid of excess heat. 

When the body’s cooling mechanisms work well, our core temperatures will drop and stabilise to a safe level. When these mechanisms fail, heat exhaustion can occur and can lead to heatstroke.  

Employees performing heavy physical work in hot or humid work environments are likely to be exposed to heat levels which could place them at risk of heat stress. These work environments include both indoor environments such as foundries, boiler rooms, mills and smelters, as well as outdoor environments such as trenching, construction, excavation, and grading tasks. Work involving hazardous wastes or asbestos requires employees to wear semi-permeable or impermeable protective clothing. This type of clothing will contain the heat generated by the body during heavy manual tasks and inhibit the body’s ability to cool itself, which significantly contributes to the employee’s exposure to heat stress.  

Various factors should be considered when assessing employees’ exposure to heat stress: 

• Thermal Environment: The thermal environment is measured by the WBGT index. The WBGT index considers three environmental factors: 

• Air temperature 
• Radiant heat 
• Cooling effects of evaporation 

The WBGT is measured using a heat stress monitor, which utilises three types of thermometers: 

• A dry bulb thermometer which measures air temperature. 
• Globe thermometer (black bulb) which measures radiant heat. 
• Wet bulb thermometer which measures the cooling effect of evaporation caused by air movement. 

It should be noted that the WBGT must not be used as an indicator of safe or unsafe work conditions as it is only an aid used to recognise overall heat stress. The following factors are also assessed: 

• Personal risk factors: Individual susceptibility makes predicting heat stress difficult. Certain physical conditions can reduce our body’s natural ability to manage high temperatures: 

• Weight: Employees who are overweight are more susceptible to heat stress as their bodies are less efficient at losing heat.  
• Poor physical condition: When we are physically fit our bodies are better equipped to cope with the increased demands that heat places on our bodies. 

• Environmental factors: Factors such as air movement, relative humidity and air temperature all affect employee’s response to heat: 

• Radiant heat: Heat transferred from hot worksurfaces or objects to the body is referred to as radiant heat. Radiant heat could also be a result of direct solar radiation (sunlight) being conducted into the work environment. 
• Humidity: Heat loss evaporation is hindered by high humidity but is helped by low humidity.  
• Air movement: If the air temperature is less than the worker’s skin temperature, increasing the rate of air movement helps to cool down the worker by increasing the rate of evaporation and the heat exchange between the skin surface and the surrounding air.  

• Job factors: Factors such as clothing or the required use of personal protective equipment and workload play a significant role in the amount of heat stress that employees will be exposed to: 

• Clothing and personal protective equipment (PPE): Clothing or PPE which is coated with or made from non-woven materials, hinder the body’s ability to evaporate sweat and cool the skin. Furthermore, dark-coloured clothing will absorb more radiant heat. 
• Workload: During more physical, heavy manual tasks, the body will generate more heat.  

When trying to reduce or control employees’ exposure to heat stress, various means can be used to achieve this: 

• Engineering controls: These controls aim to reduce the heat levels within a work environment. This can be achieved by: 

• Installing strategically positioned fans throughout a work environment to facilitate airflow. 
• Making use of a combination of mechanical interventions to draw out warm air and passive means to allow fresh cool air to enter the work area. 
• Automation of processes to reduce workload. 
• Shielding radiant heat sources from the surrounding work environment. 

• Work procedures: 

• Continuous monitoring of the thermal environment. 
• Implementing work and rest schedules. 
• Ensuring employees take sufficient breaks, have access to drinking water and ensure that they drink at least 600 millilitres of water per hour. 

• Training:  

• Ensuring that employees receive adequate training on how to identify the risk factors, signs and symptoms and health effects of heatstroke.  
• Measures that should be taken to avoid heat stress. 
• Knowledge of heat stress hazards.